The temperature dependent lifetimes, absorption, and emission spectra of an Er3+:YAG at 300–550 K are investigated and compared to the expected evolution using a theoretical description of the spectra. Population and linewidth dependent changes in the spectra can be separated, allowing conclusions on pump and laser efficiency at elevated temperatures. The temperature sensitivity of Er3+:YAG is for instance much less than that of Yb3+:YAG.